1. Field of the Invention
The present invention relates to a process for the production of colored protective coatings on articles of aluminum or aluminum alloys which have been previously anodized in a special way in order to obtain products which are particularly suitable for use in architectural applications.
2. Description of the Prior Art
Much time and attention devoted in the prior art to the production of aluminum articles which are decorative and resistant to abrasion under atmospheric influence. Early processes have included chemical coloring of aluminum articles which had previously been anodically anodized by the treatment of the same with dyes, such as aniline dyes. As the art is well aware, the thus resulting articles have poor resistance towards atmospheric influence. Other developments have included anodic oxidation of aluminum articles, followed by submersion in chemicals which penetrate into the pores of the oxide layer, so that when the thus treated aluminum article is placed in aqueous solutions of salts which also penetrate into the pores, combination with the first used chemical is possible. These processes have not proven practical for a wide variety of reasons.
It is also known in the prior art to simultaneously anodize and color aluminum articles. However, the art is aware that processes of this type result in only a limited selection of colors and that the processes are expensive and difficult to carry out and very rigid requirements are made for the working and heat treatment of the aluminum articles as the metallic structure therein is of the utmost importance for the result obtained. These simultaneous processes also demand the use of large currents and high voltages and long times and are thus relatively expensive.
U.S. Pat. Nos. 3,669,856; 3,769,180 and 3,849,263 represent recent developments in the field of coloring aluminum or aluminum alloys. These patents are, in general, directed towards the coloring of anodized aluminum by immersing said article in a bath containing a salt of a particular metal and passing an alternating current between the previously anodized article and a counterelectrode.
Although the process of these patents represent a significant improvement in the field of coloring aluminum, nevertheless, no details are given as to how the previously formed anodic coating is formed on the aluminum and, in fact, at least the implication is present that conventional anodizing techniques are used.
It is also well known in the art of anodizing aluminum that two separate and distinct types of an oxide layer can be obtained which are generally referred to in the art as a hard coat or a soft coat. The conventional anodizing techniques utilized in the art result in the production of a so-called soft coat. There are processes known in the art for the production of hard, dense anodic coatings, but the techniques employed in the art for the subsequent dyeing of these hard, dense coatings have involved the conventional immersion with a suitable dye, as opposed to an electrolytic coloring process. The reason for this might be due to the fact that the techniques for the production of hard anodized coatings result in the production of anodic layers which are significantly colored and can therefore only be dyed to darker and muddier colors by the use of organic or inorganic dyestuffs. The art is also aware that the thicker the anodic layer is formed that the darker the anodic layer will be and, in general, those processes which produced hard anodic layers had as one of their criticalities the production of a thick layer. These thick layers of anodic film are totally unsuitable for the novel process of this invention.
As has heretofore been stated, there are processes known in the art for the production of a hard anodized layer. Of such known processes, those which have the anodizing bath at low temperature (around 32.degree. F.) or intermediate temperature (around 45.degree. F.) are unsuited for the use of the novel process of this invention for several reasons. In the first place, these processes are expensive and require substantial energy in order to operate. Further, these processes produce an anodized layer which is relatively thick (customarily 1.5 mil or heavier) in order to obtain high wear resistance, and which is a darkish, muddied color thereby rendering it unsuitable for use where light, unmuddied colors are desired. On the other hand, the known hard coat process which operates at room temperature (with the anodizing bath at about 70.degree. F.) uses a higher current density than that of the present invention, resulting in the production of an anodized layer unsuitable for use in producing a wide range of colors.
U.S. Pat. No. 3,524,799 is directed towards a room temperature process for anodizing aluminum in order to produce hard, dense anodic coatings and the novel process of the present invention utilizes as one step thereof a modification of the process disclosed by this patentee.
The specification and claims of this patent are directed to the formation of hard, dense anodic coatings on aluminum or aluminum alloys by anodizing the aluminum in an aqueous electrolyte containing a mineral acid, such as sulfuric acid, a polyhydric alcohol of 3 to 6 carbon atoms, an organic carboxylic acid containing at least one reactive group in the alpha-position to the carboxylic acid group, such as lactic acid or glycine, and an alkali salt of a titanic acid complex of a hydroxyaliphatic carboxylic acid containing from 2 to 8 carbon atoms, such as, for example, titanium dilactate ammonium salt.
We have now discovered that the use of such anodizing techniques without the alkali salt of a titanic acid complex provide extremely dense and hard anodic coatings optimally suited to architectural applications and that such anodized layers when colored using the techniques described in U.S. Pat. No. 3,669,856 provide aluminum and aluminum alloy surface of very pleasing, architecturally pure colors of exceptional uniformity. Additionally, the use of the combination of these prior art techniques apparently provides exceptional throwing power in the coloring operation. Throwing power is a term of art defining the ability of a coloring bath and process to provide color uniformly to all surfaces of the surface of a workpiece undergoing coloring. Thus, a process and bath which demonstrates high throwing power provides uniform color to small creases, cracks, nooks, detents, etc., as well as the larger uniform surfaces of an aluminum or aluminum alloy workpiece being colored. High throwing power also permits the introduction into the coloring bath of a mix of workpieces in terms of their alloy composition and overall physical configuration to obtain uniform color of all such workpieces. In prior art coloring techniques it was often difficult, if not impossible to obtain uniform coloring of workpieces of different alloys or shape in a single coloring bath at the same time. Furthermore, as is well recognized by the skilled artisan in the aluminum coloring field spacing of the various workpieces in the coloring bath was a critical factor in successfully uniformly coloring aluminum extrusions, particularly for architectural purposes. Such spacing restraints often required leaving sufficient distances between the undivided pieces being colored that substantial portions of the working volume of a given coloring tank were left empty during a coloring operation resulting in very inefficient use of coloring tank capacity. The exceptional throwing power of the technique of the instant invention permits minimal spacing of the workpieces in the coloring bath and thus optimum usage of the coloring capacity of a coloring tank. This results not only in a more optimum efficiency in terms of use of tank capacity, but reduces substantially the chemical and power requirements of the electrolytic coloring process.